secp256k1-sys: remove restriction on cc version

.travis.yml

@@ -21,6 +21,7 @@ matrix:
     os: windows
 
 script:
+  - cargo generate-lockfile --verbose && cargo update -p cc --precise "1.0.41" --verbose
   - cargo build --verbose --no-default-features
   - cargo build --verbose --no-default-features --features="serde"
   - cargo build --verbose --no-default-features --features="lowmemory"

secp256k1-sys/Cargo.toml

@@ -19,7 +19,7 @@ links = "rustsecp256k1_v0_1_1"
 features = [ "recovery", "endomorphism", "lowmemory" ]
 
 [build-dependencies]
-cc = ">= 1.0.28, <= 1.0.41"
+cc = "1.0.28"
 
 [lib]
 name = "secp256k1_sys"

secp256k1-sys/README.md

@@ -26,6 +26,11 @@ $ ./vendor-libsecp.sh depend <version-code> <rev>
 - Where `<version-code>` is the secp256k1-sys version number underscored: `0_1_2`.
 - Where `<rev>` is the git revision of libsecp256k1 to checkout.
 
+## Note
+
+The build dependency `cc` might require a recent version of the rust compiler.
+If you want to compile it with rust 1.22.0, pin its version in your `Cargo.lock`
+with `cargo update -p cc --precise 1.0.41`.
 
 ## Linking to external symbols